The present disclosure relates generally to the use of transient voltage suppression (TVS) diodes to protect aircraft power control circuitry and their components from transient power surges. More specifically, the present disclosure relates to shared TVS diode-based protection schemes that facilitate the efficient and cost effective protection of solid state power controller (SSPC) components from exposure to overvoltage conditions caused by lightning-induced pulses and other transient events, while also facilitating the efficient and cost effective incorporation of built-in-test (BIT) circuitry for TVS diode failure points.
Vehicles, such as aircraft, typically include miles of wires and dozens of computers and other instruments and systems that control everything from the engines to passenger headsets. One or more power management and distribution (PMD) systems are typically provided to distribute power from a primary or secondary source to various vehicle systems. PMD systems often include so-called smart power management and distribution functionality enabled by SSPCs. In addition to providing protection for cable harnesses and loads, smart SSPCs are capable of accurately monitoring power quality and load conditions, which permit the PMD system controller to react to power fluctuations and faults automatically and in real time. SSPCs also provide wide programmability, which allows PMD systems to adapt to system reconfiguration and future equipment insertion. A typical PMD system may include hundreds or thousands of SSPCs.
Aircraft computers and electrical systems, including PMD systems and their SSPCs, must be able to safely withstand overvoltage conditions and other transients that can result from a lightning strike. Traditionally, aircraft had an aluminum skin that attenuated the lightning current induced on the wires. Some aircraft now use composite materials instead of aluminum for weight and strength benefits. However, composite materials do not provide the same level of attenuation to lightning as aluminum. When lightning occurs, hundreds of volts may surge between a load in the vehicle system and the aircraft chassis. As such, the lightning requirements of PMD systems and their SSPCs have increased.
SSPCs may use microprocessors to manage the operation of high-efficiency switching MOSFETs, which perform on/off control of the load and protect loads from short circuit and overload conditions. MOSFET gate drives can be designed to control the rise and fall time of channel currents. When these MOSFETs are subjected to lightning-induced power surges and other transients that are higher than the MOSFET voltage ratings and they are OFF, the MOSFETs break down and conduct, which typically results in the MOSFET being damaged or destroyed.
It is known how to use TVS diodes to protect the MOSFET switches of an SSPC from lightning-induced power surges and other transients. TVS diodes provide protection to MOSFETs by shunting excess current when the lightning-induced voltage exceeds the TVS diode avalanche breakdown potential. TVS diodes are, in effect, clamping devices that suppress all voltages above their breakdown voltages, and they automatically reset when the overvoltage goes away. A TVS diode may be either unidirectional or bidirectional. A unidirectional TVS diode operates as a rectifier in the forward direction like any other avalanche diode but is made and tested to handle very large peak currents. The bidirectional TVS diode clamps in both directions and can handle very large peak currents in both directions.
Known TVS diode-based protection schemes for SSPC MOSFET switches require an individual TVS diode for every SSPC output channel. As the complexity of SSPCs for aircraft applications increases, a single SSPC card can include 40 or more output channels, which, following known transient protection schemes, would require 40 individual TVS diodes per SSPC card. Additionally, although TVS diodes have sufficient functionality to provide the necessary transient protection, known TVS designs exhibit dormant failures. Existing SSPCs that utilize TVS diode-based protection transient protection schemes typically do not have a way of testing the full functionality of its protection circuitry without removing the module containing the protection circuitry from the aircraft itself or without the use of an excessive amount of BIT circuitry. Either option is not desirable. As a result, the functionality of the protection circuitry is typically determined during maintenance and assumed to be maintained until the next maintenance. Verification is then performed at the next maintenance when the module is removed from the aircraft. Hidden or dormant TVS diode failures are not immediately evident to operations and maintenance personnel as soon as they occur, so the detection of such dormant failures require a specific action (e.g., a periodic application of BIT circuitry) in order for the dormant failure to be identified. Providing the necessary BIT circuitry to test 40 or more TVS diodes per SSPC card for dormant failures is complicated, large, expensive and typically cost prohibited.
It is therefore desirable to provide a TVS diode-based protection scheme that facilitates the simple, efficient and cost effective protection of SSPC power channels and components from exposure to overvoltage conditions caused by lightning-induced pulses and other transient events, while also facilitating the efficient and cost effective incorporation of BIT circuitry for identifying dormant TVS diode failures.